This invention relates to a balancing system for improving the balance of an applied d-c voltage across a string of series-connected, generally similar circuit components, such as capacitors, where the circuit components have different I-V (current versus voltage) characteristics and different leakage resistances which prevent natural voltage sharing or equal division.
It is particularly important to balance the d-c voltage across a series string of capacitors which provide the capacitance of an LC filter at the output of a large A-C to D-C power converter to which is applied a-c line voltage. In a large A-C to D-C power converter (for example, where 460 volts three-phase a-c is converted to about 620 volts d-c), the filter usually includes a number of capacitors connected in series to provide a sufficient voltage rating, and then at least one additional series string of the same number of capacitors is connected in parallel with the first string in order to obtain the required capacitance. Although the individual capacitors in the series/parallel bank will ordinarily be of similar construction, having the same capacitance and voltage rating, their leakage resistances, and hence leakage currents, will likely differ. This is especially true when the capacitors are of the electrolytic type. In the absence of any balancing arrangement, the leakage current differences between the capacitors could cause a large voltage unbalance across the capacitors In other words, if the leakage resistances are different, the d-c voltage across a series string of capacitors will not divide or be shared equally by all of the capacitors The voltage across one of the capacitors could be sufficiently high to exceed the capacitor's voltage rating, thereby damaging or destroying the capacitor.
The traditional method in the past of balancing the voltage (or more accurately reducing the voltage imbalance) across a series string of capacitors is to place a resistor in parallel with each set of capacitors, a set comprising the corresponding individual capacitors in each series string which will be parallel connected. In effect, each set of parallel capacitors constitutes one capacitor The resistor added across each set is substantially smaller than the parallel combination of the leakage resistances in the set so that the resistor will determine the effective resistance of the set. As a result, the effective resistances of the various sets, or composite capacitors, may be somewhat equalized. Unfortunately, these additional resistors, shunting the capacitors, dissipate a substantial amount of power. The larger the possible mismatch in leakage current, the smaller the value of the resistors that must be used to force voltage balancing, and the greater the power loss
The balancing system of the present invention is a substantial improvement over those previously developed in that voltage balancing across a series string of capacitors is achieved by means of a network which dissipates a very small amount of power compared to the prior systems.